Hitherto, cinnamic acid (abbreviated as CA hereinafter) has been produced by a process for reacting benzaldehyde with acetic anhydride in the presence of potassium or sodium acetate, that is, a so-called Perkin process (Organic Reactions, Vol. 1, 217(1942)). The said process is an established one but it has defects that it employs a large amount of expensive acetic anhydride and potassium or sodium acetate, requires a precipitating process with an acid and a recrystallization process after the reaction, and produces waste water.
On the other hand, a process for the production of CA by the liquid-phase oxidation of cinnamic aldehyde (abbreviated as CAL hereinafter) prepared by the condensation reaction of benzaldehyde with acetaldehyde in an aqueous caustic soda solution using molecular oxygen in the presence of silver oxide catalyst is a publicly-known process disclosed in U.S. Pat. No. 3,162,682 and in British Pat. No. 782430. The said process has, however, defects that it employs expensive silver oxide as the catalyst and requires the reaction temperature to be maintained at 30.degree. C. or lower (because a silver mirror reaction occurs, causing a catalyst loss, if its reaction temperature exceeds 30.degree. C.). In addition, the said process requires a precipitating process with an acid and a recrystallization process and also produces waste water, as in the Perkin reaction.
The inventors have devoted themselves to studies on a process for preparation of CA by oxidation of CAL which does not have the above-mentioned defects and as a result, have found that, if CAL dissolved in an aromatic hydrocarbon is subjected to a liquid-phase oxidation reation employing molecular oxygen in a reaction system consisting of the said solution and coexisting water in the presence of a cobalt compound catalyst, CAL is converted into CA at a high yield, the reaction system is separated into an aqueous layer and an oil layer, and high-purity CA crystals are obtained readily by cooling the oil layer. Further, they have found that the said aqueous layer contains the catalyst dissolved in it and can be recycled for use and thus they have completed the invention.
The invention is a process for the production of cinnamic acid by the oxidation reaction of cinnamic aldehyde characterized in that cinnamic aldehyde dissolved in an aromatic hydrocarbon is oxidized in a liquid phase using molecular oxygen in the coexistence of a cobalt catalyst and water.